A typical aircraft engine fuel system includes a fuel supply arrangement that includes a boost pump, and a supply pump. The supply arrangement provides an outlet flow of fuel that may be used in one or more metering circuits each of which are responsible for metering fuel in a combustion engine, and/or an outlet flow of fuel that may be used for other functions, e.g. actuator control. In the context of metering circuits, the metering of fuel in each metering circuit is achieved via a fuel metering valve (FMV). The control of the fuel metering valve is achieved via an electro-hydraulic servo valve (EHSV). In the context of actuator control, actuator positions are controlled via an EHSV.
Each EHSV receives an input supply of fuel which is used to hydraulically position the EHSV (also referred to herein as a “servo”). This input supply of fuel is provided from the supply pump discharge flow. Each servo also drains the input supply of fuel back the inlet side of the supply pump. This recirculation is often referred to as servo drainage. A conventional configuration of such an arrangement is shown at FIG. 8. As shown therein, a boost pump 800 is connected in series to a supply pump 802. First and second servos 804, 806 are connected in parallel to one another to the outlet of supply pump 802. Supply pump 802 provides an outlet flow at pressure Ps which then passes through a wash screen 808 such that a portion of the outlet flow from supply pump 802 is split between servos 804, 806 and is at pressure Psf. The flow passing through servos 804, 806 then drains to the inlet side of supply pump 802 at pressure Pb.
Servos 804, 806 may be indicative of separate EHSV's responsible for controlling the metering position of one or more FMV's 810. As can also be seen at FIG. 8, a discharge pressurizing valve 812 is connected to an outlet side of FMV 810. Also, a bypass valve 814 is responsible for bypassing an unused portion of the fuel flow provided by supply pump 802 to metering valve 810 back to the inlet side of supply pump 802 at pressure Pb.
Unfortunately, as the number of various servos increases, so too does the overall leakage of the system. In order to ensure sufficient supply pump outlet flow characteristics to support engine relight, the supply pump must be sized to provide an output sufficient to accommodate a relight event as well as accommodate for the total servo leakage of the system. Where an overall high servo leakage is present, an oversized supply pump is often required. An oversized supply pump can lead to undesirable pump heat input to the fuel system, as any unused flow is recirculated. This recirculation heats the fuel and can negatively impact the engine's thermal management system, as well as lead to significant fuel inefficiencies.
Accordingly, there is a need in the art for an aircraft engine fuel system that substantially reduces or eliminates the need for such an oversized supply pump by more efficiently managing total servo leakage.
The invention provides such a system. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.